Method of correcting an erroneous frame by a receiver

ABSTRACT

The present invention relates to a method of processing a received data unit (UDR) by a receiver via a network, the method comprising a channel decoding stage (CDEC1, CDEC2) of the received data unit (UDR), intended to deliver a hard bit string (TBD), a transformation stage for transforming said hard bit string into a hard frame (TD), said hard frame comprising at least an error detector code and a test stage (TST) intended to test with said error correcting code if said hard frame (TD) is correct or incorrect. Said method is characterized in that it further comprises a frame correction stage (COR) intended to correct an incorrect frame (TE) into a correct frame (TCo). In the preferred embodiment of the invention said frame correction stage (COR) utilizes soft data delivered by a soft output channel decoding stage (CDEC2). The advantage of such a method is that rejects and retransmissions of frames are avoided.

The present invention relates to a processing method for a data unitreceived by a receiver, the method comprising a channel decoding stageof the received data unit, intended to deliver at least a string of hardbits, a transformation stage of said hard bit string into a hard frame,said hard frame comprising at least an error detecting code and a teststage intended to test on the basis of said error detecting code if saidhard frame is correct or incorrect.

The invention also relates to a receiver comprising means forimplementing such a method.

The invention also relates to a transmission system comprising such areceiver.

The invention also relates to a computer program which implements such amethod when it is executed by a processor.

The invention finally relates to a signal conveying such a program.

The invention notably finds its application in data transmission via anetwork having high error rates, for example, a wireless network.

The transmission of digital data between a source application and adestination application is generally effected by means of a network asis shown in FIG. 1. Such a network is organized in layers, for example,according to the OSI reference model (Open Systems Interconnection),comprising seven layers. These layers are managed by protocols and forma stack called network stack.

When a source application SAPP sends data to a destination applicationDAPP via a network, said data are to pass through a first network stackPR₁ from top to bottom before reaching the transmission channel C andbeing transmitted in the form of a unit of transmitted data UDE. Thisfirst stack and the source application SAPP are considered to belong toa transmitter 1. A data unit UDR is then received by a second networkstack PR₂ which it is to pass through from bottom to top before beinggiven access to the destination application DAPP. The second stack andthe destination application are considered to form part of a receiver 2.

At the transmitter end as well as the receiver end the layers of anetwork stack each ensure well-defined functions. In that which followsa distinction will be made between the layers (L₁, L₂, . . . , L₇) ofthe network stack PR₁ situated at the transmitter end (L′₁, L′₂, . . . ,L′₇) from those of the network stack PR₂ situated at the receiver end.

Let us consider in particular the two first layers called base layers,that is to say:

-   -   the physical layer L₁ (L′₁ respectively) which provides the        transmission of hard data over the transmission channel C. It        generally comprises a channel coder (decoder respectively)        charged with coding (decoding respectively) the data so as to        protect them from possible disturbance during transmission;    -   the data link layer L₂ (L′₂ respectively) whose main aim is to        transmit the data of the network layer L₃ from a transmitter to        the network layer L′₃ of a receiver. On the side of the        transmitter, the data link layer L₂ converts a frame transmitted        via the network layer L₃ into a bit string which it directs to        the physical layer L₁. On the side of the receiver, the data        link layer L′₂ divides the received bit string into data frames        that have almost no errors.

To transmit the data from the network layer L₃ of a transmitter to thenetwork layer L′₃ of a receiver, the data link layer is thus to make useof the service of the physical layer L₁ (L′₁ respectively). The latterensures the transport of data units over the transmission channel C andtheir return to the network stack PR₂. On the side of the transmitter,the physical layer L₁ comprises a channel coder which converts a bitstring into a data unit UDE. The data unit UDE is sent over thetransmission channel C. On the side of the receiver a received data unitUDR is converted into a received bit string by a channel decoder.

Such a bit string may have errors. Such errors have various causes, forexample, an attenuation of the signal transmitted over the transmissionchannel, or noise or also a congestion of the network. The number ofreceived bits may be lower than or equal to or higher than the number oftransmitted bits. The value of certain bits may also have changed. Inorder to be able to detect possible transmission errors at the receiverend, the data link layer L₂ at the transmitter end generally addserror-detecting codes to the data it receives from the network layer L₃.From these codes, from received data and from other control codes itconstructs at least one frame. With the aid of these error detectingcodes it calculates, for example, a checksum for each frame. When aframe arrives at its destination, the data link layer L′₂ at thereceiver end recalculates this checksum. If the result obtained isdifferent from that calculated by the transmitter, the data link layerknows that a transmission error has occurred. It may then take measuressuch as rejecting the received frame and sending a negative settlementto the transmitter. In that case the transmitter will be able toretransmit the frame, for example. However, it happens that only severalbits of the received frame are corrupted. In that case the fact that theretransmission of said frame is requested is particularly costly interms of overloading the network and processing time and this inrelation to the low number of erroneous bits.

On the other hand, if the result is identical, the data link layer atthe receiver end takes the received frame into account and may send apositive acquittal to the transmitter.

It should be noted that very often the physical layer L₁ itself addserror correcting codes to a data unit before sending it over thetransmission channel C. Such correcting codes are used on the side ofthe receiver by the physical layer L′₁ to detect and correcttransmission errors. Contrary to the error detecting codes which decidewith a very low error margin if a frame is correct or incorrect, theerror correcting codes correct a data unit so as to produce therefrom aversion corrected in the best possible way as regards said codes. Thecorrected data unit is then offered to the channel decoder whichdelivers a bit string.

It should finally be noted that for reasons of transmission costs thedata link layer L₂ or L′₂ almost never utilizes error correcting codesexcept in the particular case of a one-way channel, thus does not permitthe retransmission of an erroneous frame.

These detecting codes and error correctors are described in the thirdedition (ISBN 2 10004315 3) of the book entitled “Networks” written byAndrew Tanenbaum, edited by Dunod in the collection “Prentice Hall”,particularly pages 192 to 198.

It is an object of the present invention to propose a solution forcorrecting an erroneous frame, the solution comprising at least an errordetecting code at the data link layer of the receiver so as to avoid thetransmitter retransmitting said erroneous frame.

This object is achieved by the method as described in the openingparagraph and characterized in that it comprises a frame correctionstage intended to correct an incorrect frame into a correct frame, saidstage comprising:

-   -   a sub-stage of selecting bits to be corrected in an incorrect        frame, from data extrinsic to said incorrect frame,    -   a sub-stage of generating a candidate frame from bits to be        corrected and from the incorrect frame, said candidate frame        being intended to be tested by the test stage.

An advantage of the method according to the invention is that the numberof retransmissions of erroneous frames is limited and thus the networkis not uselessly crowded.

The selection sub-stage for bits to be corrected selects in theerroneous frame the bits of which the probability of being erroneous islargest. Such a selection is made, for example, from data which arepresent at the receiver end, which are for example a priori knowledge ofthe coding technique of the data on the side of the transmitter or fromdata generated by the receiver from the received data unit.

The candidate frame generation sub-stage generates a candidate frame bymodifying bit values to be corrected.

The candidate frame obtained is then tested by the test stage. If saidstage declares the candidate frame to be correct, the candidate framereplaces the erroneous frame by the candidate frame that is sent to theupper layer of the network stack, that is to say, the network layer.

In a first embodiment of the invention said frame correction stage isrepeated as long as the candidate frame generation stage delivers a newcandidate frame and as long as the new candidate frame is declared to beerroneous by said test stage. An advantage of this embodiment is that itpermits to test various candidate frames, which increases the chances ofcorrecting the erroneous frame. On the other hand, it is necessary toprovide an end for said repetitions in order to avoid blocking theprocessing of data units received by the receiver. In other words, thisis about establishing a compromise between retransmission of erroneousframes and processing time by the receiver.

It is for this reason that in a second embodiment of the invention theframe correction stage further includes a check sub-stage intended tocount a number of candidate frames and to stop the generation and thetest of candidate frames if said number reaches a predeterminedthreshold. An advantage of this second embodiment is that the number ofrepetitions of the correction stage for erroneous frames is limited. If,after a certain number of attempts, no candidate frame has been declaredcorrect by the test stage, the erroneous frame is rejected and theretransmission may be requested.

In a preferred embodiment of the invention the method comprises achannel decoding stage of the received data unit, intended to deliver ahard bit string and a soft bit string and a transformation stage fortransforming said hard bit string into a hard frame and said soft bitstring into at least one soft frame, said soft frames forming theextrinsic data of the hard frame. Said soft frames produce in effect fora given bit of the erroneous hard frame probability that this bit isexact. Such a probability is advantageously used during the sub-stage ofselecting bits to be corrected.

The invention also relates to a receiver implementing the methodaccording to the invention.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiment(s) described hereinafter and to the accompanying drawings inwhich:

FIG. 1 describes a transmission system for transmitting data via anetwork;

FIG. 2 is a block diagram of the data link layer of a network stackaccording to the prior art;

FIG. 3 is an example of a frame delivered by the sub-layer MAC of thedata link layer in the case of the Ethernet standard;

FIG. 4 is a block diagram of the correction method of an erroneous frameaccording to the invention;

FIG. 5 is a block diagram of the method according to the inventioncomprising a stage of controlling the number of generated candidateframes;

FIG. 6 is a block diagram of the method according to the inventioncomprising a soft output channel decoding stage and a stage of checkingthe number of generated candidate frames; and

FIG. 7 describes hard bit string and soft bit string structures producedby the channel decoding stage.

FIG. 2 shows the data link layer L₂ of a network stack. On the side ofthe transmitter, such a layer comprises a transformation stage E_TRANSof transmitted data DE via the network layer L₃ into a transmitted frameTE. This transformation stage E_TRANS consists of adding check codes tothe data DE. The transmitted frame TE is converted into a bit stringTBE, then coded by the channel encoder CENC which delivers a unit UDE oftransmitted data. On the side of the receiver, a received data unit UDRis decoded by a channel decoder CDEC which delivers a received bitstring TB. A symmetrical transformation stage R_TRANS then consists offinding back at least one frame called received frame TR in saidreceived bit string TB. Let us consider the particular case of a localarea network or LAN utilizing the Ethernet standard (or IEEE 802.3).Such a network comprises a transmission channel, also called broadcastchannel, which is bi-directional. In this context the data link layer L₂or L′₂ comprises a sub-layer of channel access control which is calledMAC sub-layer (Medium Access Control) and is notably in charge ofmanaging the potential user access to the transmission channel C. Saidsub-layer also comprises the transformation stage R_TRANS mentionedpreviously. FIG. 3 shows an example of frame TR extracted by the MACsub-layer from a received bit string. Such a frame first comprises aheader of 14 octets and formed by three fields:

-   -   a destination address of the frame TR;    -   an address of origin of the frame TR;    -   a type of data describing the frame TR.

The frame finally comprises useful data transmitted by the sourceapplication SAPP and finally error detecting codes CRC.

The data link layer L′₂ on the side of the receiver also comprises atest stage TEST intended to test whether the received frame TR iscorrect or incorrect. This stage has for its object to detect possibleerrors which are present in the received frame TR and to realize this itdepends on the error detecting codes CRC which have been added to thetransmitted data DE by the data link layer L₂ on the side of thetransmitter. In practice, polynomial codes are often used, also calledcyclic redundancy check codes CRC. In the polynomial codes is consideredthat the bits of a character string are the coefficients of apolynomial. For example, the string 110001 comprises 6 bits and itrepresents a polynomial of 6 terms whose coefficients are 1, 1, 0, 0, 0,1 and are equal to x⁵+x⁴+x⁰. To utilize a polynomial code thetransmitter and the receiver are to be in agreement with a choice of thepolynomial generator G(x).

To calculate the checksum mentioned earlier of a block of m bitscorresponding to a polynomial M(x) and longer than G(x), the principleconsists of affixing check bits to the end of the block so that theframe (block and check bits) can be divided by G(x). When the receiverreceives a frame, it divides the frame by G(x). If the rest obtained isnot zero, there is a transmission error.

In that case the frame is rejected. The link layer L′₂ of the networkstack PR₂ of the receiver may request a retransmission of the frame inquestion to the link layer of the network stack PR₁ of the transmitter.

An example of the method according to the invention is shown in FIG. 4.It is characterized in that it further comprises a frame correctionstage COR intended to correct an erroneous frame TE. It is an object ofthis stage to notably avoid rejecting the erroneous frame and itspossible retransmission.

It should be well noted that this frame correction stage COR is situatedat the level of the data link layer and that it is applied to receivedframes called erroneous frames by the test stage TEST. It is at everypoint different from the data unit correction stage mentioned above,which may be applied to a received data unit at the physical layer bymeans of error correction codes.

The frame correction stage COR comprises a sub-stage of selecting bitsto be corrected BC in the erroneous frame TE from extrinsic data IS ofthe frame, available at the receiver end. The stage consists ofselecting a certain number of bits to be corrected BC in the erroneousframe TE. The bits to be corrected BC are for example those bits thathave the greatest probability of being erroneous. Time to correct allthe bits of the erroneous frame TE would in effect be prohibitivelycostly in terms of calculation time. It is thus an object of thissub-stage to limit the number of bits to be corrected and thus thecomplexity of the method.

The extrinsic data IS of the erroneous frame TE relate, for example, toa priori knowledge of an unequal protection technique of the data(unequal error protection) implemented by the transmitter. Indeed, inthis framework certain data present in the erroneous frame have beenmore protected than others and it may be supposed that the bits formingsuch data have less probability of being erroneous.

The candidate bits BC are then presented to a candidate frame generationsub-stage GENER intended to form a candidate frame T_(Ca) from candidatebits BC and from the erroneous frame TE. Such a sub-stage depends onpattern generator techniques such as the Chase algorithm or thethreshold pattern generator algorithm.

Let us consider an erroneous frame TE to be a bit sequence y=(y₁, y₂, .. . , y_(L)) with L being an integer greater than zero. The principle ofthe Chase algorithm is the following: let us suppose that the sub-stageof selecting bits to be corrected has selected k bits, with k being aninteger greater than zero, as those having the largest probability ofbeing erroneous. The Chase algorithm constructs N patterns in thefollowing way:

-   -   one candidate bit is set to 1 and all the others to zero;    -   2 candidate bits are set to 1 and all the others to zero;    -   i candidate bits are set to 1 and all the others to zero;    -   p≦k candidate bits are set to 1 and all the others to zero.        There are thus $N = {\sum\limits_{i = 1}^{p}C_{k}^{i}}$        possible patterns.

A pattern M_(i) is then applied to the erroneous frame TE in thefollowing manner:S_(i)=M_(i)⊕y,where ⊕ represents the binary OR-exclusive function (XOR). Thisoperation narrows down to inverting in the erroneous frame the candidatebit or candidate bits which has or have been set to 1 in the pattern.

The new obtained frames, also called candidate frames, are then arrangedaccording to a metric which measures a distance to the erroneous frame,the first candidate frame being nearest to the erroneous frame. In thecase where an unequal protection of data has been used at thetransmitter end, such a distance may be calculated by applying differentweights to the inverted bits depending on the protection they havereceived. The candidate frames arranged such will then be testedaccording to the set order.

The operation of this algorithm may be influenced by the two parametersk and p. The parameter k determines the maximum number of bitsconsidered faulty in the erroneous frame and the parameter p the numberof bits that may be tried to be corrected from the k candidate bits. Theparameter p depends on the error correcting codes used and notably theirlength as well as the transmission as such. It is generally chosen to beless than k/2.

The threshold pattern generator algorithm is at many points similar tothe Chase algorithm. It is distinguished by a unique parameter θ insteadof k and p. In the candidate bit selection sub-stage all the bits arechosen whose probability of being exact also called trust bits is lessthan θ. The rest is identical to the Chase algorithm with a minorreservation that p=k, which corresponds to the number of bits below thetrust threshold θ.

It should be noted that for the method according to the invention it isonly necessary for the frames delivered by the data link layer L₂ tocomprise one or various error detecting codes. The frame correctionstage according to the invention under no circumstance needs detectingcodes which are moreover error correctors. In the presence of errorcorrecting codes the method according to the invention only needs thesecodes for detecting errors.

The first candidate frame T_(Ca) obtained is then tested by the teststage TEST. If said stage declares it to be correct, the candidate frameT_(Ca) replaces the erroneous frame TE and the correct frame T_(Co) issent to the upper layer of the network stack PR₂, that is to say, to thenetwork layer L′₃. The data link layer L′₂ can thus transmit a positiveacquittal to the destination of the data link layer of the network stackPR₁ of the transmitter.

If, on the other hand, the first candidate frame T_(Ca) is declarederroneous by the test stage TEST, the second candidate frame is testedin its turn. In a first embodiment of the invention such a process isrepeated until all the generated candidate frames are tested or oneframe is declared correct. If no candidate frame is satisfactory, thedata link layer L₂ sends a negative settlement to the transmitter byasking for a possible retransmission of the erroneous frame.

With the Chase algorithm we have seen that it was possible to define thenumber of bits to be corrected by the parameters p and k and thus tochoose a desired number of candidate frames T_(Ca). The number ofrepetitions is thus known in advance and does not vary as a function ofthe received frames. On the other hand, the threshold pattern generationalgorithm operates differently. It utilizes a trust threshold θ as thebits to be corrected have a probability of being exact lower than thisthreshold. The number of bits to be corrected thus varies with thereceived frames, for example, as a function of transmission conditions.Thus it could happen that the number of candidate frames generated bysuch an algorithm is very high because many bits have a probability ofbeing exact that is lower than the threshold θ, which would render thecost of correction prohibitive. This is the reason why in a secondembodiment of the invention the error frame correction stage COR furthercomprises a check sub-stage CONT intended to count a number of candidateframes and stop the test of candidate frames if said number reaches apredetermined threshold. An advantage of this second embodiment is thatthe number of repetitions of said correction stage COR is limited andnot the processing time of received frames is increased too much.

It should be noted that also a minimum threshold could be introduced toguarantee a minimum number of candidate frames.

The check sub-stage CONT is shown in FIG. 5. It comprises a countingfunction COUNT intended to count the number nb of candidate framesalready tested and a comparing function STOP of comparing this number nbto a threshold, intended to stop the correction process if the number nbis higher than the predetermined threshold nbmax. In this case it issuitable to adapt the choice of (k,p) or of θ to nbmax so as to avoidgenerating useless frames in the candidate flame generation sub-stageGENER.

In what has been stated above, no hypothesis has been made as to animportant stage of the physical layer L′₁, which is the channel decodingCDEC. The object of such a stage is to convert a transported data unitin the form of a physical signal by the transmission channel C intodigital data also called bit string. The physical signal received by thereceiver comprises real values that the channel decoding stage CDEC isto convert into bits. There are mainly two types of channel decodingalgorithms: the simpler one associates a binary value equal to 0 or 1 toa real value of the physical signal, also called binary decision. Suchan algorithm is called “hard” at the output. The second associates tosaid real value not only a binary value but also a trust measure whichexpresses the probability that the decision is exact. This secondalgorithm is called “soft” algorithm at the output. In this second casethe channel decoder delivers not only a string of hard bits but also astring of soft bits comprising quantized values of trust measures.

The method according to the invention as described previously relates toa hard frame TD delivered by the transformation stage R_TRANS of thedata link layer L₂ from a hard bit string TBD coming from the physicallayer L₁. Said hard bit string TBD is formed by decisions made by thechannel decoding stage CDEC of the physical layer L₁. Only the “hard”outputs of the channel decoding stage CDEC are thus used by thetransformation stage R_TRANS.

In the preferred embodiment of the invention shown in FIG. 6 this time asoft output channel decoding stage is considered. The object is toutilize the trust measures associated with the decisions delivered bythe channel decoding stage for the correction of an error frame. Moreexactly, said trust measures are intended to be taken into account bythe selection sub-stage SEL of bits to be corrected for the choice ofbits that have the greatest probability of being erroneous.

Let us consider a real physical signal received by the physical layer L₁comprising L real values, with L being a non-zero integer. Let ussuppose that the channel decoding stage at the “soft” output hasdelivered a binary decision h_(i) and a trust measure c_(i) for a realvalue i of said physical signal, where i is situated between 1 and L. Inorder to be transmitted from the physical layer L₁ to the data linklayer L₂, said trust measure c_(i) is to be quantized by a quantizationsub-stage of the channel decoding stage CDEC₂, according to a techniqueknown to the expert and intended to deliver N−1 bits (s_(i,2), . . . ,s_(i,N)) with N being an integer greater than or equal to 1. As is shownin FIG. 7, a real value i of a physical signal received by the channeldecoding stage CDEC₂ is thus represented by N bits, N≧1 (s_(i,1),s_(i,2), . . . , s_(i,N-1)). Said quantization sub-stage thus delivers astring of hard bits TBD and a string of soft bits TBS, said soft bitstring being constructed by concatenating binary representations of theL real values over N−1 bits.

Said soft bit string TBS is shown in FIG. 7. The most significant softbits (s_(i,1), s_(2,2), . . . , s_(2,L)) are placed first, then the lesssignificant soft bits and so on up to the least significant soft bits(s_(N,1), s_(N,2), . . . , s_(N,L)).

It should be noted that the soft bit string TBS is then transmittedwithout any difficulty from the physical layer L₁ to the data link layerL₂. The physical layer and the data link layer are indeed often combinedto a single layer and the exchange of data between them is facilitatedbecause of this.

The hard bit string TB and the soft bit string TBS are then received bythe transformation stage R_TRANS. The not very restrictive hypothesis ismade that said transformation stage knows the number of bits N utilizedfor the quantization. Also knowing the length of the bit string TBS thetransformation stage is then capable of dividing said bit string intoN−1 sections of equal size T_(n), n being situated between 1 and N−1.From the section T_(n) it thus constructs a soft frame TS_(n). From thehard bit string TBD it constructs a hard frame TD.

The hard frame TD is then transmitted to the test stage TEST. If saidhard frame TD is declared correct, it is sent to the network layer L′₃and a positive settlement is sent to the transmitter. If, on the otherhand, said hard frame is declared erroneous, it is transmitted to theframe correction stage COR.

The erroneous bard frame TE is received by the selection sub-stage SELfor bits to be corrected, as are the N−1 soft frames Ts_(n) which areassociated thereto. These N−1 soft frames constitute the extrinsic dataIS intended to be used by the sub-stage SEL for selecting bits to becorrected. In a simple way the sub-stage SEL for selecting bits to becorrected may find back the trust measures contained in the N−1 softframes Ts₁ and use these trust measures for selecting the bits of thehard frame that have the largest probability of being erroneous, forexample, with the aid of one of the two algorithms that have been shownabove.

As has been seen previously, the bits to be corrected BC are transmittedto the sub-stage GENER of generating a candidate frame. This sub-stagemay also advantageously utilize the trust measures delivered by thechannel decoding stage at the “soft” output, to put the candidate framesin the order from the most probable to the least probable. For example,it may be considered that a candidate frame where only the bit that hasthe greatest probability of being erroneous has been modified, is nearerto the sought frame than a candidate frame obtained from modifying a bitthat has less probability of being erroneous.

The advantage of the latter embodiment is that additional data, the softdata available at the level of the physical layer L′₁, can be benefitedfrom to better correct the erroneous frames of the data link layer L′₂.

The invention is not restricted to the embodiments that have just beendescribed by way of example. Modifications or improvements may beapplied while remaining within the scope of the invention.

The description above with reference to FIGS. 1 to 7 illustrates theinvention rather than limits same. It is evident that there are otheralternatives that remain within the framework of the appended claims.

There are many ways of implementing the functions described by means ofsoftware. With respect thereto FIGS. 1 to 7 are very diagrammatic, eachFigure representing only one embodiment. Thus, although a Figure showsvarious functions in the form of separate blocks, this does not excludethat a single software item carries out various functions. This does notexclude either that one function can be carried out by a software set.

It is possible to implement these functions by means of a receivercircuit comprising one or various processors, said processors beingsuitably programmed. A set of instructions contained in a program memorymay cause the circuit to carry out various operations described earlierwith reference to FIGS. 1 to 7. The set of instructions may also beloaded in the program memory by reading a data carrier, for example, aCD-ROM. Reading may also effected via a communication network such asthe Internet network. In that case a service provider will put a set ofinstructions at the disposal of interested parties.

No reference sign in brackets in a claim is to be interpreted inlimitative fashion. The verb “to comprise” does not exclude the presenceof other elements or stages and those listed in a claim. The word “a” or“an” preceding an element or a stage does not exclude the presence of aplurality of these elements or stages.

1. A processing method for a data unit received by a receiver via anetwork, the method comprising a channel decoding stage (CDEC₁, CDEC₂)of the received data unit (UDR), intended to deliver at least a stringof hard bits (TBD), a transformation stage (R_TRANS) of said hard bitstring (TBD) into a hard frame (TD), said hard frame comprising at leastan error detecting code and a test stage (TEST) intended to test on thebasis of said error detecting code whether said hard frame (TD) iscorrect or incorrect, said method further comprising a frame correctionstage (COR) intended to correct an incorrect frame (TE) into a correctframe (T_(co)), said stage comprising: a sub-stage (SEL) of selectingbits to be corrected (BC) in an incorrect frame (TE), from data (IS)extrinsic to said incorrect frame; a sub-stage (GENER) of generating acandidate frame (T_(ca)) from bits to be corrected (BC) and theincorrect frame (TE), said candidate frame being intended to be testedby the test stage (TEST).
 2. A processing method as claimed in claim 1of a data unit received by a receiver, characterized in that saidchannel decoding stage (CDEC₂) of the received data unit (UDR) isintended to deliver a soft bit string (TBS) and in that saidtransformation stage (R_TRANS) is intended to transform said soft bitstring (TBS) into at least a soft frame (TS_(n)), said soft frame(TS_(n)) being intended to be used as extrinsic data (IS) of theincorrect frame (TE).
 3. A processing method as claimed in claim 1 of adata unit (UDR) received by a receiver, said frame correction stage(COR) being repeated as long as the candidate frame generation sub-stage(GENER) delivers a new candidate frame (T_(Ca)) and the new candidateframe (T_(Ca)) is declared erroneous by said test stage (TEST).
 4. Aprocessing method as claimed in claim 3 of a data unit (UDR) received bya receiver, further comprising a check sub-stage (CONT) intended tocount a number of candidate frames (nb) and to stop the frame correctionstage (COR) if said number reaches a predetermined threshold (nb_max).5. A receiver intended to process a received data unit (UDR), comprisingchannel decoding means (CDEC₁, CDEC₂) of the received data unit (UDR),intended to deliver at least a hard bit string (TBD), transformationmeans (R_TRANS) for transforming said hard bit string (TBD) into a hardframe (TD), said hard frame comprising at least an error detecting code,and test means (TEST) intended to test on the basis of said errordetecting code whether said hard frame (TD) is correct or incorrect,said receiver further comprising frame correction means (COR) intendedto correct an incorrect frame (TE) into a correct frame (T_(Co)), saidmeans comprising: means (SEL) for selecting bits to be corrected in theincorrect frame (TE) based on data (IS) extrinsic to the frame;generation means (GENER) for generating a candidate frame (T_(Ca)) basedon bits to be corrected (BC) and on the incorrect frame (TE), saidcandidate frame (T_(Ca)) being intended to be tested by the test stage(TEST).
 6. A receiver as claimed in claim 5, characterized in that saidchannel decoder (CDEC₂) is intended to deliver a soft bit string (TBS)and in that said transformation means (R_TRANS) are intended totransform said soft bit string (TBS) into at least one soft frame(TS_(n)), said soft frame (TS_(n)) being intended to be used asextrinsic data (IS) to the incorrect frame (TE).
 7. A transmissionsystem comprising a transmitter intended to transmit a data unit via anetwork and a receiver intended to process a received data unit (UDR),said receiver comprising channel decoding means (CDEC₁, CDEC₂), of thereceived data unit intended to deliver at least a hard bit string (TBD),transformation means (R_TRANS) for transforming said hard bit string(TBD) into a hard frame (TD), said hard frame comprising at least anerror detecting code and test means (TEST) intended to test on the basisof said error detecting code whether said hard frame (TD) is correct orincorrect, said receiver further comprising frame correction means(COR), intended to correct an incorrect frame (TE) into a correct frame(T_(Co)), said means comprising: means (SEL) for selecting bits to becorrected in the incorrect frame (TE) based on data (IS) extrinsic tothe frame; generation means (GENER) for generating a candidate frame(T_(Ca)) based on bits to be corrected (BC) and on the incorrect frame(TE), said candidate frame (T_(Ca)) being intended to be tested by thetest stage (TEST).
 8. A transmission system as claimed in claim 7,characterized in that said channel decoder (CDEC₂) is intended todeliver a soft bit string (TBS) and in that said transformation means(R_TRANS) are intended to transform said soft bit string (TBS) into atleast a soft frame (TS_(n)), said soft frame (TS_(n)) being intended tobe used as extrinsic data (IS) to the incorrect frame (TE).
 9. Acomputer program for a receiver comprising a set of instructions forimplementing a method as claimed in claim 1 when said program isexecuted by a processor.
 10. A signal intended to convey a computerprogram as claimed in claim 9.